1. Field of the Invention
The present invention relates to a hot-melt adhesive composition comprising polyethylene wax as processing aid and intended for coating sheetlike structures such as textiles, leather, foam materials or plastic films.
2. Discussion of the Related Art
Hot-melt adhesive compositions are known which are applied preferably in the form of an aqueous paste in a pattern to a sheetlike structure that is to be coated, followed by drying and sintering. The sintered product constitutes the hot-melt adhesive composition. The coated sheetlike structures, in the form of webs or cut blanks, are laminated with other sheetlike webs or blanks, under the anion of pressure and comparatively mild temperature, to form laminates of particularly high washing stability and adhesion.
Numerous hot-melt adhesive compositions for the screen printing technique are known. They are composed predominantly of aqueous-pastelike mixtures of fine hot-melt adhesive powders which, even in relatively coarse particle form, can be applied per se directly for coating by the scatter or powder dot technique. The hot-melt adhesive compositions most commonly employed today are based on copolyamides, copolyesters or high-density polyethylene.
For pasting up the fine hot-melt adhesive powders to prepare the adhesive layer, use is made of special suspending or dispersing agents, which give the pasted-up formulations cleaner printing properties. In addition, dispersing agents should also bring forth particular attributes, examples being the prevention of sedimentation of the mixed-in powders, crisply contoured print formation, controlled penetration of the hot-melt adhesive paste, prevention of outer fabric strikethrough, avoidance of strikeback in the course of fixing, or the elimination of the phenomenon of edges sticking together when the sheetlike structures are cut to size. Examples of such dispersing agents have been described, for example, in the patents DE-B 20 07 971, DE-B 22 29 308, DE-B 24 07 505, DE-B 25 07 504 and DE-B 35 10 109. Plasticizers can be added to the dispersing agents. In many cases, additives comprising high molecular mass polyethylene oxide are also added, the intention being thereby to improve the flow properties of the paste.
When preparing hot-melt adhesive powders, stearates (of Ca, Mg and Zn) are added in order to improve removal from the screen and to take account of performance requirements. With the aid of these products, it is possible to vary the bulk density and free-flow properties of the powders. Amorphous fine powder fractions (from 0 to 80 .mu.m), however, tend to form lumps even when the amount is relatively large (0.5%).
However, in connection with the processing of low-melting, amorphous polyester and polyamide hot-melt adhesives of low viscosity there is a known coating problem, namely excessive tack after the coated inliner leaves the drying oven. When considering the tack, or tackiness, of a coated surface, a distinction is made between permanent tack and temporary tack. Temporary tack is the time until recrystallization, i.e. the time until the adhesive falls below the melting temperature. The result is sticking to the calender walls (scatter coating) or sticking together of the individual plies on the winder, which in the course of further processing leads to great difficulties (material showing, lengthening).